Endless flexible power transmission media, in the form of chains or belts, have been widely used in the timing drive systems of engines, and also as for power transmission in industrial machinery and the like. Tensioners are used to prevent vibration in these chains and belts, and, at the same time, to take up slack resulting from their elongation. The tensioner typically presses against a tensioner lever, which is in sliding contact with the chain or belt. One well-known type of tensioner, known as a ratchet-type tensioner, is described in Japanese Patent Nos. 3226029 and 3226030.
An example of a conventional ratchet-type tensioner is shown in FIGS. 7 and 8. As shown in FIG. 7, the tensioner 51, which is attached to an engine body (not shown), imparts tension to a timing chain 56, which is wrapped around a driving sprocket 53 on the crankshaft 52 of the engine, and a driven sprocket 55 fixed to a camshaft 54. The tensioner presses a pivotable tensioner lever 60 against the slack side of the chain 56.
In the ratchet type tensioner 51, a plunger 58 protrudes slidably from the front of the tensioner housing 57, and an end 58a of the plunger 58 pushes against a back surface of a pivoted tensioner lever 60 at a location remote from the lever's pivot 59, so that a shoe 61 of the lever 60 is pressed against the chain 56.
As shown in FIGS. 7 and 8, a plunger-accommodating hole 62 is formed in the tensioner housing. The plunger 58 fits slidably into the plunger-accommodating hole. A hollow portion 63, formed inside the plunger 58, has an opening facing the bottom of the plunger-accommodating hole 62. A spring 64 is installed between the bottom of the plunger-accommodating hole 62 and the bottom of the hollow portion 63. The spring 64 biases the plunger 58 in a direction to protrude from the tensioner housing 57.
A pawl 66 is pivoted on a ratchet shaft 65 fixed to the tensioner housing. The pawl 66 has ratchet teeth 66a engageable with ratchet teeth 58b formed on a side of the plunger 58.
A biasing force is imparted to the pawl by a spring 67 in compression between the ratchet 66 and the tensioner housing 57. This force urges the pawl in a direction such that its teeth 66a engage the ratchet teeth 58b on the plunger. The engagement between the pawl teeth 66a and the ratchet teeth 58b blocks retraction of the plunger 58.
Since the main components of the conventional ratchet type tensioner comprise the tensioner housing 57, the plunger 54, the plunger spring 64, the pawl spring 67 and other parts, the total number of parts is large. The number of parts is even greater in the case of a ratchet-type hydraulic tensioner, where the plunger and the tensioner body cooperatively form a high pressure oil chamber which is in communication with the engine lubricating oil through a one-way check valve.
In addition, in the production of the tensioner it is necessary to form the plunger-accommodating hole 62 and a hole in the housing for accommodating the pawl spring 67. Furthermore, a hollow portion 63 must be formed in the plunger 58, and ratchet teeth 58b must be formed on the exterior of the plunger 58. Manufacture of these elements requires a large amount of time and labor, resulting in a high cost.
In the operation of a ratchet-type tensioner, until the plunger 58 is advanced by one ratchet tooth so that the pawl 66a climbs over a next tooth 58b of the ratchet 66, forward movement of the plunger may be followed by a limited amount of backward movement. Thus, a significant amount of backlash can occur in the case of a ratchet-type hydraulic tensioner, resulting in vibration of the plunger, the tensioner lever, and the chain or belt. Such vibration can also occur in a ratchet-type hydraulic tensioner, especially upon starting of an engine, when oil under pressure is not supplied immediately to the high pressure oil chamber of the tensioner.
Objects of this invention are to solve the above-mentioned problems, to provide a tensioner having a simple structure quite different from that of a conventional tensioner, to provide for easier manufacture, and to suppress backlash more effectively.